Imaging process

ABSTRACT

A layer of a suspension of electrically photosensitive particles in an insulating carrier liquid is coated onto a transparent, conductive electrode. A second electrode in roller configuration is caused to traverse the free surface of the suspension while an electrical field is imposed across the suspension between the two electrodes. The imaging suspension is illuminated imagewise ahead of the nip formed by the roller electrode which causes the illuminated electrically photosensitive particles to be drawn into the nip while the particles in dark areas are squeezed off the transparent, conductive electrode. On completion of roller traverse a reversal image is found adhering to both electrodes. By reversal image is meant that electrically photosensitive particles deposit on the electrodes in light struck areas. The process may also be used to increase the efficiency of electrophoretic deposition for conventional photoelectrophoresis. For this purpose uniform illumination is used ahead of the nip during the deposition step causing a majority of the particles to be drawn into the nip thus increasing the final density of images formed by photoelectrophoresis.

United States Patent 1191 Harris i 1 IMAGING PROCESS [75] Inventor: Lee B. Harris, Rochester, NY.

[73] Assignee: Xerox Corporation, Stamford,

Conn.

[22] Filed: Apr. 23, 1973 [21] Appl. No.: 353,837

Primary E.raminerNorman G. Torchin Assistant Examiner-John L. Goodrow Auurm'y, Agent, or Firm-James J. Ralabate; David C. Petre; Richard A. Tomlin 1 Sept. 16, 1975 l 5 ABSTRACT A layer of a suspension of electrically photosensitive particles in an insulating carrier liquid is coated onto a transparent, conductive electrode. A second electrode in roller configuration is caused to traverse the free surface of the suspension while an electrical field is imposed across the suspension between the two electrodes. The imaging suspension is illuminated imagewise ahead of the nip formed by the roller electrode which causes the illuminated electrically photosensitive particles to be drawn into the nip while the particles in dark areas are squeezed off the transparent, conductive electrode. On completion of roller traverse a reversal image is found adhering to both electrodes. By reversal image is meant that electrically photosensitive particles deposit on the electrodes in light struck areas.

The process may also be used to increase the efficiency of electrophoretic deposition for conventional photoelectrophoresisv For this purpose uniform illumination is used ahead of the nip during the deposition step causing a majority of the particles to be drawn into the nip thus increasing the final density of images formed by photoelectrophoresis.

18 Claims, 3 Drawing Figures IMAGING PROCESS BACKGROUND OF THE INVENTION This invention relates in general to imaging systems. More specifically, the invention concerns a photoelec trophoretic imaging system.

There has been developed a photoelectrophoretic imaging system capable of producing monochromatic or polychromatic images in a one pass (simultaneous exposure and development) mode which utilizes electrically photosensitive particles in a liquid suspension. the process is described and claimed in US. Pat. No. 3,383,993 to Yeh; US. Pat. No. 3,384,565 to V. Tulagin and L. Carreira; US. Pat. No. 3,384,566 to H. E. Clark and US. Pat. No. 3,384,488 to V. Tulagin and L. Carreira all issued May 21. 1968 the disclosures of which are incorporated herein by reference.

In a preferred embodiment of the process described in those patents, electrically photosensitive particles are dispersed in an insulating carrier liquid providing an imaging suspension. The imaging suspension is placed between a transparent conductive electrode referred to as the injecting electrode and an electrode having an insulating outer surface referred to as the blocking electrode. A field is applied across the imaging suspension and the suspension is exposed to an image of electromagnetic radiation, conventionally visiblc light to which the particles are responsive. As these steps are completed. selective particle migration takes place in image configuration providing a visible image normally on both electrodes. The particles apparently undergo a net change in charge polarity upon exposure to activating electromagnetic radiation when brought into interaction range with the injecting electrode. The particles when exposed to radiation to which they are sensitive apparently accept the charge of the injecting electrode and are repelled by it migrating to the blocking electrode leaving behind a positive image on the surface of the injecting electrode and forming a complementary negative or reversal image on the blocking electrode surface. The particles which move to the blocking electrode are less able to exchange charge with the insulating surface of the blocking electrode and do not cycle within the system. The insulating surface also aids in supporting the relatively high fields used in the process.

To provide monochromatic images, all the particles may be of the same color, also, the suspension for monochrome imaging may contain particles of many colors, the final image being a combination of the colors where desired. For forming images of more than one color, two or more differently colored particles are utilized each being sensitive to radiation in separate wavelengths with relatively little overlapping response and approximately equal photosensitivity. For full color subtractive imaging. a particularly preferred embodiment utilizes cyan pigment particles responsive mainly to red light, magenta pigment particles responsive mainly to green light. and yellow particles respon sive mainly to blue light.

Because images are formed by the subtraction of particles from the transparent conductive substrate the density of the images depends on the amount of particles which remain behind on the transparent electrode which is a function of the amount of particles which were there originally. Also for monochrome imaging, background free images may be formed on the blocking electrode surface, only if the contact between the blocking electrode surface and undesired particles can be minimized.

SUMMARY OF THE INVENTION It is an object of this invention to provide a photoelectrophoretic imaging system which overcomes the abovenoted disadvantages.

It is another object of this invention to provide relatively improved photoelectrophoretic deposition.

It is another object of this invention to provide a new imaging system based on photoelectrophoretic principles.

It is another object of this invention to provide a new imaging system based on photoelectrophoretic principles.

It is another object of this invention to provide a photoelectrophoretic imaging system which provides relatively low background monochrome images.

The above objects and others are accomplished in accordance with one embodiment of this invention by providing a photoelectrophoretic imaging system wherein an imaging suspension comprising electrically photosensitive particles in a carrier liquid is placed be tween a transparent conductive electrode and a second roller electrode. The roller electrode is rolled across the imaging suspension and the imaging suspension is exposed to imagewise radiation just in front of the nip; which is the area of closest approach between the roller electrode and the transparent conductive electrode. The imaging suspension is exposed to imagewise radiation to which the particles are responsive just in front of the nip. As the roller is pushed across the suspension most of the liquid and particles in dark areas of the suspension are pushed off of the transparent conductive member. In light struck areas, however, the electrically photosensitive particles which are responsive are drawn into the nip and adhere to both electrodes.

It is not fully understood why the particles which are struck by radiation to which they are sensitive are drawn into the nip. It is known that the area of strongest electrical field is in the area of closest approach between the electrodes. One theory which has been proposed is that the radiation causes the particles to become highly polarized. If the illuminated particles have an apparent dielectric constant which is greater than that of the carrier liquid they will be drawn into the areas of highest field strength. A second theory is that the particles exchange charge with each other or otherwise take on one sign of charge and are then drawn to the electrode of opposite polarity.

In either case, reversal images are formed on both electrodes. This is in marked contrast to the more conventional photoelectrophoretic imaging process as shown in the above cited patents where a positive image is formed on one electrode and a negative image is formed on the opposite electrode. This shows the necessity for keeping light away from the nip area since it would destroy at least one of the images.

The size of the nip can vary greatly depending on the flexibility of the roller electrode or the transparent conductive electrode. In a further example the roller electrode can be in the form of a web entrained over two or more rollers to form a tractor type electrode. This electrode would be particularly suitable where it is desired to use the image formed on the blocking electrode side as the final image. The nip forming surfaces may also be two webs as shown in US. Pat. No. 3,723,288, issued Mar. 27, 1973 to J. W. Weigl. This patent shows the conventional photoelectrophoretic process wherein illumination can be made other than through a conductive transparent electrode. in U.S. Pat. No. 3,723,288 the nip is exposed to radiation causing the formation of a positive and a negative image. The exposure step in the present process may also be made other than through an electrode eliminating the use of transparent materials however the illumination must be prevented from exposing the nip area.

Further, the blocking electrode side may be transparent and illumination made through it eliminating the need for utilizing transparent conductive materials.

The essential requirements of the present process are that there be a well-defined nip formed and that illumination occur just ahead of the nip but not within the nip and that the contact between the nip forming surfaces be such that the unilluminated imaging suspension is squeezed out of the nip. Normally this would entail a small amount of force being applied between the electrodes. in a cyclic operation such as where a roller is caused to traverse a flat plate, the imaging suspension is merely pushed off the image areas onto an edge of the plate where it may be washed away and reusedv In an operation which is continuous, for example where the electrodes are continuous webs, some liquid removal may be desired to control the amount of liquid which piles up ahead of the nip.

The electrically photosensitive particles may comprise any suitable electrically photosensitive particles known from the photoelectrophoresis art and may be of more than one component. Typical electrically photosensistive particles include those listed in US. Pat. No. 3,384,488. Typical particles include particles of organic pigment materials such as quinacridones, carboxamides such as N-2"-pyridyl 8,l3, dioxodinaphtho-( 2,1-b', 2',3'd) furan-6-carboxamide; triazines; benzopyrocolines; anthraquinones azos such as l-(3'- pyrenylazo)-2-naphthol', phthalocyanines both metalfree and metal-containing, and inorganic materials such as cadmium sulfide; cadmium sulfoselenide; zinc oxide; zinc sulfide; sulphur; selenium; mercuric sulfide; lead oxide; lead sulfide; cadmium selenide, titanium dioxide; indium trioxide and mixtures thereof. The particles may be of more than one component and may be dye sensitized to alter their spectral response. The X- form of metal-free phthalocyanine as shown in US. Pat. No. 3,357,989 to J. F. Byrne et al is preferred because of its high response.

It is desirable to use particles which are relatively small in size because smaller particles produce more stable suspensions with the carrier liquid and are capable of producing images of higher resolution than would be possible with particles of larger sizes. The photosensitive particle should preferably be less than 5 microns in diameter with a size range of from about 0.5 to about 5 microns being preferred although particles up to about microns may readily be used.

The carrier liquid may comprise any suitable insulating material which may be liquid or a solid which may be converted to a liquid at the time of particle migration. Typical insulating materials include decane, dodecane, N-tetradecane, kerosene, molten paraffin, molten beeswax or other molten thermoplastic material, mineral oil, silicone oils such as dimethylpolysiloxane and fluorinated hydrocarbons. Mineral oil is preferred (ill because of its high insulating qualities and because it is relatively nonvolatile.

The concentration of electrically photosensitive particles in the imaging suspension may vary over a wide range depending on the nature of the constituents, the particle size and other conditions. conventionally from about one part by weight to about twenty parts by weight of electrically photosensitive particles are used per 100 parts by weight carrier liquid.

It is also possible to provide the imaging suspension in the form of a solid layer which is converted to a liquid suspension at or prior to the time of imaging by application of heat or solvent. Such layers may be formed by dispersing photoresponsive particles in a resin solution, coating the solution on a substrate and allowing the solvent to evaporate. Typical resins include materials such as Piccotex and substituted styrene copolymer resins available from Pennsylvania Industrial Chemical Co., Staybelite Esters 5 and 10 hydrogenated resin esters available from Hercules Powder Co., Amberol ST-l37-X available from Rohm and Haas, eicosane, ceresin and similar organic waxes and mixtures thereof. Typical solvents for these binder materials include those listed as carrier liquids.

Images may be produced in one or more colors as desired, for example, color separation images may be used as the input image and the two or more particulate images formed may be transferred to a single member in register forming as many colors as desired. Also the process may be used for one step full color imaging using a color negative original as input.

lt has also been found surprisingly that uniform illumination of the imaging suspension just ahead of the nip can increase the efficiency of electrophoretic deposition. Electrophoretic deposition is used prior to the imaging step in photoelectrophoretic imaging to drive the electrically photosensitive particles into close proximity or actual contact with the injecting electrode. The deposition generally provides two advantages in conventional photoelectrophoretic imaging. For monochrome imaging the desired image is normally formed on the blocking electrode. By driving the majority of the particles to the injecting electrode only clear liquid contacts the blocking electrode except where illuminated particles migrate to it thus greatly reducing back ground. For one Step subtractive polychrome imaging the desired image is normally formed on the injecting electrode. By driving the particles originally to the injecting electrode images of higher density and color saturation are provided without having to overload the pigment suspension. Conventional electrophoretic deposition processes are shown for example in copending applications Ser. No. 863,608, now abandoned and 863,506, now U.S. Pat. No. 3,645,874. both filed Oct. 3, 1969.

To increase the efficiency of electrophoretic deposition the imaging suspension is exposed to radiation to which the particles it is desired to deposit are respon' sive. For example for full color imaging, white light would be used for deposition. The actual image formation would take place as a separate step subsequent to the deposition step. Where monochrome images are desired, any desired monochrome color could be reproduced by starting with a full color imaging suspension and using light of a chosen range of wavelength to deposit particles corresponding to the final image color desired.

BRIEF DESCRIPTION OF THE DRAWINGS The advantages of this method of photoelectrophoretic imaging will become apparent upon consideration of the detailed disclosure of the invention, particularly when considered with the accompanying drawings wherein:

FIG. I is a schematic side view ofa simple exemplary system for using the process of this invention as an imaging process.

FIG. 2 is a schematic side view of a simple exemplary system for using the process of this invention to electrophoretically deposit imaging particles and to form a conventional photoelectrophoretic image.

Referring now to FIG. I, there is seen a transparent conductive electrode generally designated 1, which in this exemplary instance is made up of optically transparent glass 2 overcoated with a thin optically transparent, conductive tin oxide layer 3. Such electrodes are available commercially, e.g. from the Pittsburgh Plate Glass Company under the name NESA glass. This electrode may be in drum, web, roller or other convenient configuration.

On the surface of electrode 1, there is coated a layer 4 of finely divided particles of electrically photosensitive material 5 in an insulating carrier liquid 6. To apply field and to squeeze off the imaging suspension layer, there is provided a second electrode generally desig nated 7 in roller configuration. Again, this electrode may have any convenient configuration such as a drum, web or plate. Roller 7 has conductive central core 8 and insulating outer surface 9. The conductive center is connected to source of high DC. potential II. The opposite terminal of potential source 11 is connected to surface 3 and to ground.

In operation. suspension 4 is coated onto electrode 1. Roller 7 is then caused to traverse electrode 1 with field applied between roller 7 and electrodee I. Potential applications of from 1.000 to 3,000 volts is conventional using apparatus as shown. The space between roller 7 and electrode I is on the order of I mil or less providing field strengths in excess of 1,000 volts per mil across the suspension in the nip area.

As roller 7 traverses the suspension, narrow slit I3 is moved with roller 7 so that light I5 is allowed to illuminate the suspension only in area 16 which area repre sents the suspension aand particles being squeezed off of electrode 1. The slit illumination is modified imagewise by the original which is to be copied here repre sented as transparent sheet 17 having raised image areas 19 on its surface. In areas of the suspension which are exposed to radiation through slit [3 and transparency I7, particles 5 are drawn into the nip and adhere both to electrodes 7 and l. Optionally, in order to decrease the background, electrode generally designated 2] having an insulating outer surface 22 on conductive roller 23 is used. This roller traverses the image remaining on electrode 1 while the suspension is exposed to imagewise radiation. Roller 21 removes a small amount of particles from electrode I reducing background. The image remaining on electrode I may then be transferred to for example. paper and fixed thereon.

It should be pointed out that when electrode 7 has traversed the imaging suspension the surface of elec trode l is virtually dry. which may make it desirable to add carrier liquid after roller 7 has traversed but before electrode 21 has traversed the suspension. Also, it is not absolutely essential that rollers 7 and 21 have insulating outer surfaces but they are preferred to help support the high fields used in the system. Also, an insulating layer may be interposed between electrode I and suspension 4 and the system will be operative.

Obviously, many other arrangements may be utilized to provide imagewise illumination of area 16. Further, where there is a relatively large distance between moving slit 13 and bead 16 collimated light is preferred to avoid illumination of the nip area. In general, area 16 is from 0.001 inch to 1.0 inch ahead of the nip area depending on the specific structure of the electrodes.

Referring now to FIGv 2A, apparatus similar to FIG. I may be used. Roller 37 constructed similar to roller 7 is used to traverse imaging suspension 34 containing electrically photosensitive particles 35 in liquid carrier 36 coated on transparent conductive electrode 31. During roller 37 traverse with field applied, area 46 is exposed to uniform illumination through moving slit 43 which causes particles struck by light to which they are sensitive to be drawn into the nip depositing on electrodes 37 and 31. The particles on electrode 37 may be reclaimed and recycled where desired. Electrode 31 with particles 35 adhering to it is now ready for use in conventional photoelectrophoretic imaging as shown in FIG. 2B. Again, it may be necessary to add some carrier liquid before imaging.

Referring now to FIG. 2B electrode 3I with a uniform dense layer of particles 35 adhering to it is exposed to radiation 45 which is modulated imagewise by transparency 47 having image areas represented here by raised dark areas 49. As roller 51 traverse is made with potential applied. particles 35 struck by light to which they are responsive exchange charge with electrode 3I and migrate to the surface of electrode 51 leaving a positive image behind. Although the drawings for purposes of clarity show typical monochrome imaging, polychrome suspensions may be utilized and polychromatic imagewise light may be used to form polychrome images as explained above.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The following Examples further specifically illustrate the improved photoelectrophoretic imaging system provided by this invention. Parts and percentages are by weight unless otherwise indicated. All of the Examples are carried out in apparatus of the general type illustrated in the Figures. A 500 watt quartz iodine lamp is used to illuminate a black and white" transparency the image being projected by a lens through a moving slit inch wide through a NESA glass substrate onto the area approximately one-tenth inch in front of the area of closest approach of the roller and NESA glass surface. The imaging suspensions are prepared by dispersing the electrically photosensitive particles in mineral oil and milling the suspension until most of the particles are less than a few microns in cross section and are stably suspended.

A source of high potential is connected to the cores of the roller blocking electrodes which cores are a conductive steel roller about two inches in diameter. A three-sixteenth inch thick polyurethane layer is bonded to the steel cores to form a blocking layer. A paper sheet is placed over the polyurethane layer to act as a receiver sheet. The other lead of the source of high potential is connected to the tin oxide surface of the injecting electrode and to ground. The imaging suspen sion is coated onto the injecting electrode to a thickness of about microns. The roller electrodes are rolled across the imaging suspension at a rate of about two inches per second with field applied while the suspension is exposed to illumination.

In the following Examples, Examples l-lll demonstrate the use of the process for directly making images. Example lll demonstrates the use of the process for electrophoretic deposition.

EXAMPLE 1 An imaging suspension is prepared by milling about six parts by weight of metal-free phthalocyanine in about 100 parts by weight mineral oil.

The suspension is coated on the conductive surface of a NESA glass plate. The roller electrode is rolled across the imaging suspension with a potential of about 3000 volts D.C. applied, the roller being positive with respect to the NESA glass plate. As the roller traverses the imaging suspension, the area just ahead of the area of closest approach between the roller and the suspension is illuminated imagewise as shown in the drawing using a negative black and white transparency as the input image. On completion of roller traverse, a cyan image is found adhering to both the NESA electrodce and the paper on the blocking electrode. The images are both positive images having cyan areas corresponding to clear areas on the transparency.

EXAMPLE ll The experiment of Example I is repeated except that a second roller at 3,000 volts is passed across the NESA immediately after the first roller without illumination. This step reduces the background of the image formed on the NESA glass plate.

EXAMPLE Ill An imaging suspension is prepared as in Example I and coated on the conductive surface of a NESA glass plate. A first roller traverse is then made as in Example I with the exception that the black and white" trans 'parency is omitted. On completion of roller traverse, a uniform deposition of particles on both the NESA glass and the roller is found. A second roller pass is then made using a roller wet with mineral oil while the particles on the NESA glass are exposed to an image using the black and white transparency, thc moving slit being omitted. On completion of roller traverse. a reversal positive image is found adhering to the roller electrode and a negative image corresponding to the transparency is found adhering to the NESA glass. That is dark areas of the transparency appear as cyan areas on the NESA glass.

Although specific components and results have been described in the above Examples. other suitable materials as listed above may be used with similar results. In addition. other materials may be added to, for example, the imaging suspension or electrically photosensitive particles to synergize, enhance or otherwise modify their properties. For example. the photosensitive particles may be charge transfer sensitized to alter their electrical response.

Other modifications and ramifications of the present invention will occur to those skilled in the art upon a reading of the present disclosure. These are intended to be included within the scope of the invention.

What is claimed is:

l. A method for photoelectrophoretic imaging which comprises the steps of:

a. providing a layer of an imaging suspension comprising electrically photosensitive particles in an insulating carrier liquid on a first member;

b. forming a liquid nip of said imaging suspension between said first member and a second nip forming member;

c. exposing said suspension ahead of said nip to a pattern of electromagnetic radiation to which at least a portion of said particles are responsive, wherein the suspension within the nip is not exposed to said radiation; and

d. applying an electrical field between said first and second members whereby there is formed on the surface of each member an image which is the reverse in image sense of the pattern of radiation to which the suspension was exposed.

2. The method of claim 1 wherein said exposure is directed at an area from about 0.001 to about 1 inch ahead of said nip.

3. The method of claim 1 wherein said suspension is exposed to illumination through a nip forming member.

4. The method of claim 1 wherein said exposure is projected onto said suspension other than through a nip forming member.

5. The method of photoelectrophoretically depositing electrically photosensitive particles which comprise the steps of a. providing a layer of a suspension of electrically photosensitive particles in an insulating carrier on a first member;

b. forming a liquid nip of said suspension between said first member and a second nip forming member;

c. exposing said suspension ahead of said nip to electromagnetic radiation to which at least a portion of said particles are responsive, wherein the suspension within the nip is not exposed to said radiation; and

d. applying an electrical field between said first and second members until at least a portion of said particles are drawn into said nip and deposited on the surface of each of said members.

6. The method of claim 5 wherein said exposure is directed at an area from about 0.001 to about 1 inch ahead of said nip.

7. The method of claim 5 wherein said suspension is exposed to illumination through a nip forming member.

8. The method of claim 5 wherein said exposure is projected onto said suspension other than through a nip forming member.

9. The method as defined in claim I wherein said particles have a diameter less than about 5 microns.

10. The method as defined in claim 1 wherein one of said first and second nip forming members is a blocking electrode.

11. The method as defined in claim 10 wherein said suspension includes a plurality of differently colored electrically photosensitive particles.

12. The method as defined in claim ll wherein said suspension includes cyan, magenta. and yellow electrically photosensitive particles.

13. The method as defined in claim 5 wherein said particles have a diameter less than about 5 microns.

ticles which is formed by the method of claim 5; and

b. applying an electrical field across said imaging layer between said electrode and a second electrode wherein at least one of said electrodes is transparent and exposing said imaging layer to an imagewise pattern of activating electromagnetic radiation through said transparent electrode whereby an image is formed.

18. The method as defined in claim 17 wherein one of said electrodes is a blocking electrode.

* IF I? UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTION PATENT NO. 3, 905,812

DATED September 16, 1975 INVENTOR(S) Lee H. Harris It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 12, "the" should read The Column 2, line 7, "abovenoted" should read above-noted Claim 5, line 10, after "to" and before "elec-" insert uniform Signed and Scaled this seventeenth Day Of February 1976 [SEAL] A tteSf.

C. MARSHALL DANN Commissioner of Parents and Tradenmrks RUTH C. MASON Arresting Officer 

1. A METHOD FOR PHOTELECTROPHORETIC IMAGINE WHICH COMPRISES THE STEPS OF: A. PROVIDING A LAYER OF AN IMAGING SUSPENSION COMPRISING ELECTRICALLY PHOTOSENSITIVE PARTICLES IN AN INSULATING CARRIER LIQUID ON A FIRST MEMBER, B. FORMING A LIQUID NIP OF SAID IMAGING SUSPENSION BETWEEN SAID FIRST MEMBER AND A SECOND NIP FORMING MEMBER, C. EXPOSING SAID SUSPENSION AHEAD OF SAID NIP TO A PATTERN OF ELECTROMAGNETIC RADIATION TO WHICH AT LEAST A PORTION OF SAID PARTICLES ARE RESPONSIVE, WHEREIN THE SUSPENSION WITHIN THE NIP IS NOT EXPOSED TO SAID RADIATION, AND D. APPLYING AN ELECTRICAL FIELD BETWEEN SAID FIRST AND SECOND MEMBERS WHERENY THERE IS RORMED ON THE SURFACE OF EACH MEMBER AN IMAGE WHICH IS THE REVERSE IN IMAGE SENSE OF THE PATTERN OF RADIATION TO WHICH THE SUSPENSION AS EXPOSED.
 2. The method of claim 1 wherein said exposure is directed at an area from about 0.001 to about 1 inch ahead of said nip.
 3. The method of claim 1 wherein said suspension is exposed to illumination through a nip forming member.
 4. The method of claim 1 wherein said exposure is projected onto said suspension other than through a nip forming member.
 5. The method of photoelectrophoretically depositing electrically photosensitive particles which comprise the steps of a. providing a layer of a suspension of electrically photosensitive particles in an insulating carrier on a first member; b. forming a liquid nip of said suspension between said first member and a second nip forming member; c. exposing said suspension ahead of said nip to electromagnetic radiation to which at least a portion of said particles are responsive, wherein the suspension within the nip is not exposed to said radiation; and d. applying an electrical field between said first and second members until at least a portion of said particles are drawn into said nip and deposited on the surface of each of said members.
 6. The method of claim 5 Wherein said exposure is directed at an area from about 0.001 to about 1 inch ahead of said nip.
 7. The method of claim 5 wherein said suspension is exposed to illumination through a nip forming member.
 8. The method of claim 5 wherein said exposure is projected onto said suspension other than through a nip forming member.
 9. The method as defined in claim 1 wherein said particles have a diameter less than about 5 microns.
 10. The method as defined in claim 1 wherein one of said first and second nip forming members is a blocking electrode.
 11. The method as defined in claim 10 wherein said suspension includes a plurality of differently colored electrically photosensitive particles.
 12. The method as defined in claim 11 wherein said suspension includes cyan, magenta, and yellow electrically photosensitive particles.
 13. The method as defined in claim 5 wherein said particles have a diameter less than about 5 microns.
 14. The method as defined in claim 5 wherein one of said first and second nip forming members is a blocking electrode.
 15. The method as defined in claim 14 wherein said suspension includes a plurality of differently colored electrically photosensitive particles.
 16. The method as defined in claim 15 wherein said suspension includes cyan, magenta, and yellow electrically photosensitive particles.
 17. The method of photoelectrophoretic imaging comprising the steps of: a. providing an electrode carrying an imaging layer comprising electrically photosensitive imaging particles which is formed by the method of claim 5; and b. applying an electrical field across said imaging layer between said electrode and a second electrode wherein at least one of said electrodes is transparent and exposing said imaging layer to an imagewise pattern of activating electromagnetic radiation through said transparent electrode whereby an image is formed.
 18. The method as defined in claim 17 wherein one of said electrodes is a blocking electrode. 